dmaengine: Add note to dmatest documentation about supported channels

The documentation is not so clear for newbies in a sense of what type of the
channels are supported by it.

Clarify this by adding a note at the preamble of the documentation.

Reported-by: "Zhu, Tony" <tony.zhu@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-By: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/dmaengine/dmatest.rst

@@ -6,6 +6,11 @@ Andy Shevchenko <andriy.shevchenko@linux.intel.com>
 
 This small document introduces how to test DMA drivers using dmatest module.
 
+.. note::
+  The test suite works only on the channels that have at least one
+  capability of the following: DMA_MEMCPY (memory-to-memory), DMA_MEMSET
+  (const-to-memory or memory-to-memory, when emulated), DMA_XOR, DMA_PQ.
+
 Part 1 - How to build the test module
 =====================================